The ratio of the specific weight of a liquid to the specific weight of pure water at a standard temperature is called as:

Specific gravity of a substance is defined as the ratio of substance density to the standard substance density.

Standard substance in case of liquid is water $(\mathrm{\rho }= 1000 \mathrm{kg}/{\mathrm{m}}^3)$

Standard substance in case of gases is air $(\mathrm{\rho }= 1.02 \mathrm{kg}/{\mathrm{m}}^3)$

Specific gravity or Relative density $=\frac{\mathrm{Density} \mathrm{of} \mathrm{the} \mathrm{substance}}{\mathrm{Density} \mathrm{of} \mathrm{the} \mathrm{water} \mathrm{at} 4°\mathrm{C}}$

Specific gravity or Relative density $=\frac{\mathrm{Mass} \mathrm{of} \mathrm{any} \mathrm{volume} \mathrm{of} \mathrm{a} \mathrm{substance}}{\mathrm{Mass} \mathrm{of} \mathrm{an} \mathrm{equal} \mathrm{volume} \mathrm{of} \mathrm{a} \mathrm{water} \mathrm{at} 4°\mathrm{C}}$

Hence, the correct option is B).